BIOG 1440 Lecture Notes - Lecture 13: Partial Pressure, Pulmonary Artery, Ph
27 Apr 2018
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Gas Exchange I--gas exchange in mammals w/ emphasis on oxygen
1. Start w/ gas exchange in humans
a. Humans are aerobes, almost all energy is obtained by oxygen respiration (20.9%
in air)
2. Some important factoids about air and water (Chemistry)
a. Water is much more dense and viscous than air → oxygen content much lower
and diffusion rate much much lower (by 10000x)
b. When inhaled, some oxygen becomes a dissolved gas
i. Diffusion will cause gases to transfer into liquids
1. Relationship of gases in atm and liquid is complex
2. At equilibrium conc. In two regions are not identical
ii. Oxygen is just one of several gases in air; partial pressure exerted by gas
is its fraction of total volume
c. Gas molecules exert forces on liquids
i. One of the original ways to measure force was mmHg (760 mmHg=1 atm)
ii. Pressure
1. Diffusion of gas into liquid also depends on atmospheric pressure
a. Plunger goes down; number of atoms entering increases;
after some time equilibrium is achieved w/ more atoms in
liquid
b. If pressure increases, amount dissolved increases
i. However, solubility of gas in the given solvent is
also important
iii. Henry’s Law
1. p=kH*C
a. C=concentration of solute in sol’n (now dissolved gas)
b. p=partial pressure of solute above sol’n
c. kH=Henry’s law constatnt/coefficient; has units such as
L*atm/mol or atm/(mole fraction) or Pa*m^3/mol
2. p/kH=concentration
a. Solubility of a gas in a liquid is directly proportional to its
partial pressure
3. *Only applies to dilute solutions*
4. kH is a measure of how fast the dissolved gases want to leave the
liquid
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5. Think of op as measure of entry of gas into liquid while kH is the
converse or the measure of its entry back into the atmosphere
(dissolution)
6. If p increases, concentration will decrease; kH is not a constant--
varies with temperature; solubility of gases usually decreases as
temp increases
iv. Not all gases have the same solubility
1. CO2 >> O2 > N2
v. To add some complexity: carbon dioxide reacts w/ water to form proton
and carbonate (look at full equation)
1. If partial pressure of CO2 increases, pH decreases since rxn is
“pushed” to right. If [H+] increases, rxn is “pushed” in opposite
direction
2. Also H2CO3 is unstable so most carbonic acid formed will
become deprotonated at neutral pH
3. Respiration in the body
a. Getting Oxygen into body and carbon dioxide out
i. Mammals ventilate their lungs by negative pressure breathing, which
pulls air into lungs. Lung volume increases as rib muscles and diaphragm
contract
1. Diaphragm contracts (moves down) to inhale
2. Diaphragm relaxes (moves up) to exhale
ii. Respiratory surfaces
1. Animals require large, moist respiratory surfaces for exchange of
gases b/n cells and respiratory medium (air or water)
2. Gas exchange across respiratory surfaces takes place by
diffusion
3. Gases of interest are CO2+O2
4. Alveoli are surrounded by (covered with) capillaries
a. Alveoli have high surface area for gas exchange in lungs
iii. Gas exchange at alveolar surface (see image)
1. Most exchange is diffusion driven with active transport playing a
role with CO2 transport*
a. *sometimes active transport when CO2 leaves system
across membrane
2. [O2] is highest at lung but drops dramatically at mitochondria
a. Mitochondria uses the oxygen, so oxygen concentration
drops
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
To add some complexity: carbon dioxide reacts w/ water to form proton and carbonate (look at full equation: if partial pressure of co2 increases, ph decreases since rxn is. Inhaled air: high p(oxygen), low low p(carbon dioxide) Alveolar spaces/capillaries: 104 mmhg (oxygen), 40 mmhg (co2) Pulmonary veins and systemic arteries: same as part 2. Body tissue: <40 mmhg (o2), >45 mmhg (co2) Pulmonary arteries, systemic veins; blood entering alveolar capillaries: 40 mmhg (o2), 45mmhg (co2) Up to four oxygen molecules bound per hemoglobin. Exchange at systemic capillaries: hemoglobin loaded w/ oxygen sends oxygen to mitochondria to create atp and co2, there is a buildup of carbon dioxide due to aerobic metabolism, so it needs to be carried back to the lung. One of the most active enzymes known is a carbonic anhydrase found in erythrocytes: not present in blood plasma (not floating around free, bicarbonate accumulates in erythrocyte then moves to blood plasma.
